The invention relates to an optical delay module for lengthening the propagation path of a light beam.
The invention furthermore relates to an optical pulse multiplication or elongation module, having at least one beam splitter area, and having at least one beam combining area, and having an optical delay module of the aforementioned type.
A delay module and also a pulse multiplication or elongation module are disclosed in the document U.S. Pat. No. 5,661,748.
Delay modules and pulse multiplication or elongation modules of this type are used for example in optical beam guiding systems for semiconductor lithography. By way of example, excimer lasers that generate pulsed laser light are used as light sources in semiconductor lithography. Lasers of this type generate temporally short laser pulses, the individual length of which is approximately a few 10 ns, while the energy of the individual laser pulses is usually greater than 5 mJ. This means that the power density of the laser light is very high over the duration of an individual pulse.
These high power densities can damage downstream optical systems, for example a lithography system, or the optical components of a beam guiding system or at least shorten the service life thereof.
In order to solve the problem of the high peak powers within a laser pulse, it has therefore been proposed to divide the light beam coming from the laser into two partial beams by means of a beam splitter device and to allow one partial beam to pass through a delay module and subsequently to recombine the non-delayed light beam and the delayed light beam. In this way, it is possible to increase the pulse duration of the laser pulses, or to split each laser pulse into a plurality of temporally offset subpulses in order thus to lower the power density of each individual pulse or to reduce the peak power of the individual pulses.
The light beam generated by the laser naturally has a divergence, which has to be taken into account in pulse multiplication or elongation modules. In the case of a propagation path difference between the delayed partial beam and the non-delayed partial beam of several meters to a few tens of meters, the divergence of the light beam has the effect that the delayed partial beam has a significantly larger cross section than the non-delayed partial beam. This may have the effect that part of the light is masked out at the periphery of the light beam by optical systems arranged downstream and can thus no longer be used.
Furthermore, it is desirable for the delayed partial beam and the non-delayed partial beam or the subpulses and the original pulse all to lie on one optical axis and, as already mentioned, to have identical beam properties.
In previous delay modules and pulse multiplication or elongation modules, use is made of imaging optics that image the input of the delay module 1:1 onto the output of the delay module.
In the case of a delay module and pulse multiplication or elongation module disclosed in the document EP 1 069 453 A2 the detour line is formed by a plurality of plane mirrors, a refractive imaging optic in the form of a slightly detuned Kepler telescope being used as imaging optic for a 1:1 imaging of the input onto the output of the module. An arrangement comparable therewith is disclosed in the document U.S. Pat. No. 6,549,267 B1.
Such a pulse multiplication or elongation module has the disadvantage that the delay module requires a correspondingly large number of mirrors and optical imaging elements which all have to be separately adjusted exactly and, in addition, be correspondingly held mechanically. This makes the optical system complex, which leads to considerable costs in the production of the system and a considerable expenditure of time in adjusting the system.
In principle, in the case of the pulse multiplication or elongation module in accordance with the document U.S. Pat. No. 5,661,748 already cited in the introduction, this problem is avoided in principle by the delay module having two spherical mirrors, the radii of curvature of which are identical, and which are arranged on the common axis of symmetry with their concave sides situated opposite one another at a mirror distance from one another which approximately corresponds to the radius of curvature of the mirrors.
Through the use of two confocal spherical mirrors, the refractive imaging optic present in the known system mentioned previously can be dispensed with since the spherical mirrors already ensure a 1:1 imaging of the coupling-in area onto the coupling-out area.
In the case of this known pulse multiplication or elongation module, a beam splitter having alternately reflective and transmissive regions is used for coupling the light beam into the space between the two spherical mirrors. In this way, from the light beam coming from the laser, a totality of first beam parts spaced apart from one another are transmitted and a totality of second beam parts are coupled into the delay module. The totality of the coupled-in beam parts circulate four times in total between the two spherical mirrors and are then slightly axially offset by a beam offset plate in order then to be coupled out from the delay module by the beam splitter having the alternate transmissive and reflective sections. The delay of the totality of the coupled-in partial beams with respect to the totality of the non-coupled-in partial beams is thus essentially limited to quadruple the distance. In principle, although it would be possible to obtain greater delay lengths, further and further beam parts would always be masked out in this case, with the result that, given multiple complete circulation cycles, the light intensity decreases rapidly or the shape of the light beam is altered.
Moreover, owing to the alternately transmissive and alternately reflective beam splitter or coupling-in element, the known delay module and pulse multiplication or elongation module are tolerance-sensitive because the special beam splitter has to be adjusted exactly in relation to the offset plate, which disadvantageously increases the adjustment outlay of this known system.